To study the adequate intake (AI) for vitamin D in the elderly, we have performed an intervention study with 800 IU/d of vitamin D(3) in the institutionalized elderly. Sixty-two institutionalized elderly were randomly assigned to two groups; receiving either supplements of 200 mg calcium plus 800 IU vitamin D(3)/d (Ca+VD group), or supplements of 200 mg calcium/d (Ca group) for 30 d in October. Serum concentrations of 25-hydroxyvitamin D (25OH-D), parathyroid hormone (PTH), and bone turnover markers were measured before and after intervention. Average dietary vitamin D intake during the intervention period was approximately 300 IU/d in both groups, exceeding the AI in Dietary Reference Intakes for Japanese 2005 of 200 IU/d. In both groups, mean serum 25OH-D level at baseline fell into the hypovitaminosis D range (9.7 ng/mL), despite apparently adequate vitamin D intake. Serum PTH level at baseline was within the reference range. Mean serum 25OH-D concentration significantly increased to 19.3 ng/mL in the Ca+VD group and to 11.1 ng/mL in the Ca group. Post intervention serum 25OH-D level was significantly higher in the Ca+VD group than in the Ca group (p<0.001). In 53 subjects (85.5%) who took more than 80% of their supplements for 30 d, serum PTH level in the Ca+VD group was significantly lower than in the Ca groups (p=0.05). Bone turnover markers were not significantly changed after intervention in either group. Daily supplementation of 800 IU vitamin D(3) was considered effective in the institutionalized elderly with minimal chance of sun exposure, but further studies with longer duration are necessary.